It is known that the purification of wastewater constitutes a major problem. Accordingly, the European Union has been led to issue a directive (No. 91/271/EEC) relating to the treatment of urban wastewater which determines the limits of discharges, into the natural environment, of untreated wastewater. Thus, each treatment unit is attributed a precise objective as regards the quality of the water treatment, it being possible for failure to achieve such an objective to give rise to penalties of a financial or even a penal nature.
The majority of urban wastewater treatment plants use the activated sludge process. An important phase of this process consists in the removal of the carbon and the nitrogen contained in the wastewater, by sequencing of the periods of aeration. It is indeed known that the main problem encountered in wastewater treatment plants is adapting the treatment to the variations in the rate of entry of the water to be purified and to its polluting load, so as to obtain a constant quality of purified water and the minimum regulatory quantity of polluting discharges into the natural environment. For this purpose, the removal of carbon and nitrogen requires a very strict and precise control of aeration given that this removal must correspond to two requirements. According to the first, a sufficient total duration of aeration should be provided per day in order to carry out the oxidation of the carbon components of the wastewater and the stabilization of the sludge; the second is linked more directly to the daily distribution of the aeration phases in order to successfully carry out the removal of the nitrogen. On the one hand, it is necessary to observe a sufficient period for maintaining under aerobic conditions for the sludge to perform the nitrification and, on the other hand, the nitrification requires an appropriate effluent residence time under anoxic conditions. For this purpose, in the small-load activated sludge processes used in a single aeration basin, the removal of nitrogen compounds results from a strict control of the alternation of the aerated and nonaerated sequences.
Any defect in the setting or the operation of the oxygen supply devices results in a malfunction of the wastewater purification stations, with repercussions on the quality of the effluent treated, the equilibrium of the purifying biomass and the characteristics of the sludge produced.
A lack of adaptation of the aeration sequences therefore has effects in the short term on the quality of the water obtained which may then contain nonoxidized nitrogen compounds if the periods of aeration are not sufficiently long, or nitrates if the periods of anoxia are too short. By contrast, when the periods of nonaeration are too long, the effluent to be treated encounters anaerobic conditions which must be absolutely avoided. Indeed, the phenomena of anaerobiosis in the treatment basin, linked to an under-oxygenation of certain zones, cause in the long term the appearance of filamentous bacteria and these microorganisms induce a modification of the structure of the floc and a reduction in its sedimentation ability, which of course has an unfavourable repercussion on the quality and the cost of the treatment. Another consequence of an insufficient cumulative duration of aeration relates to the quality of the sludge and, in particular, determines its stability.
It can be understood why the regulation of aeration is one of the key points in such a water treatment process. Various methods of regulation have been used.
Thus, sensors measuring dissolved oxygen and the oxidation-reduction potential as well as various sensors serving to detect reference values have been used, a high threshold making it possible to stop the aeration and a low threshold to restart the aeration system, delaying means being used when the thresholds are not reached. These known systems are not completely satisfactory. Indeed, to optimize the nitrification and denitrification reactions, it is essential to supply oxygen when necessary and in a sufficient quantity, and not simply to supply oxygen as done by the systems described above.
In order to improve these systems, it has been necessary to continuously measure the oxidation-reduction potential EH of the medium and to analyse the go shape of the curve of the variation of this potential as a function of time. According to this known process, the derivative of this function EH=f(time) is calculated. If this derivative is negative, this corresponds to a reduction in the oxidation-reduction potential in anoxia phase and when the derivative is zero, a stabilizing phase is present. The system then calculates the period of aeration or of nonaeration to be allowed for, which is equal to the period of aeration or of nonaeration necessary to bring the oxidation-reduction potential to the value required to perform either the removal of carbon, or the nitrification, or the denitrification, plus the additional time necessary to perform the reaction. Such a system has disadvantages because the oxidation-reduction potential curve can take on an asymptotic shape (derivative tending towards 0) and, for certain oxidation-reduction potential values, it is absolutely necessary to avoid passing to a virtually stabilizing phase because the conditions are then inappropriate for the desired treatment (poor preservation of the biomass).
The present proprietor has been led to perfect the latter process. Thus, its patent FR-A-2,724,646 describes a system for regulating the aeration of a biological wastewater treatment using an activated sludge treatment applied to a combined removal of the carbon and of the nitrogen in a single basin plant provided with aeration means. Such a system recognizes, in real time, the level of treatment required in the basin and it controls the appropriate aeration sequence. Moreover, it provides a diagnostic assistance as regards the possible limits of the process. Thus, the aeration is better adapted to the conditions of the process and the reliability and the energy and economic management of the aeration are improved. It is thus possible to obtain complete removal of the carbon and of the nitrogen while maintaining a sufficient oxidation state of the sludge.
However, this system of regulation has limits. While it is perfectly suitable for activated sludge purification processes of the small load type, with a single and homogeneous aeration basin, it cannot be used on treatment sites whose configuration leads to several distinct aeration volumes or basins, placed in series, being differentiated. FIGS. 1a and 1b of the appended drawings very schematically represent two examples of such treatment plants. The cells of the aeration basins being physically separated, the oxidation state and the progress of the biological reactions in the basins are no longer uniform in the whole aeration volume. An alternation of the aerated and nonaerated sequences is still necessary in order to carry out the reactions. The known process, which was last to be described above, is not directly applicable to nonhomogeneous aeration basin configurations in series, given that the representativeness of the measurements of the oxidation-reduction potential cannot be ensured for the entire aeration volume of such basin configurations. In addition, a minimum residual carbon content is necessary in order to maintain the kinetics of denitrification at a high level. In case identical aeration sequences are maintained for all the aeration cells, the heterogeneity of the rate of progress of the reactions does not make it possible to successfully carry out and to optimize the biological reactions. It is therefore necessary to manage the aeration of the different basins in an independent and complementary manner. This is what constitutes the technical problem which is solved by the present invention.